K40, 3020 stamp engraver, 3020 laser engraver – this beast have alot of different listing names.

Typically there is three models out there on the market with three kinds of controller boards.
A personal suggestion is to only go with a Corel or Ruida version, you can find this information in the description of the listings on E-bay.

The insides of theese machines has two different gantrys, one older and the newer (K40D) appearing more common now when the older versions get sold out. V1 and V2 are not manufactured any more but the manufacturers has thousands of them still in stock and you can find them really cheap on Ebay.
There is also atleast two different cases for theese machines, one with a bit higher case, and one lower.

K40 – V1 (Analog)

Theese machines exist with Moshi and Corel controller boards.

K40 – V2 (Digital)

This version exist with both Moshi and Corel controller boards.

K40D (Digital)

This version exist with Corel and Ruida controller boards. The latter pretty uncommon.

The internals

The K40 analog and older digital looks like this on the inside.

The K40D has new internals, better gantry with sliders instead of wheels rolling. You can also recognize it by the gold-ish color, and the lack of any nozzle.
The copper pipe is my air assist addon

Wich one should i buy?

This is mostly personal preferences, and what you will use the machine for.
K40 laser buyers guide